Supercooling means the phenomenon that a molten object or a solid is not changed although it is cooled to a temperature below the phase transition temperature in an equilibrium state. A material has a stable state at every temperature. If the temperature is slowly changed, the constituent elements of the material can follow the temperature changes, maintaining the stable state at each temperature. However, if the temperature is suddenly changed, since the constituent elements cannot be changed to the stable state at each temperature, the constituent elements maintain a stable state of the initial temperature, or some of the constituent elements fail to be changed to a state of the final temperature.
For example, when water is slowly cooled, it is not temporarily frozen at a temperature below 0° C. However, when water enters a supercooled state, it has a kind of quasi-stable state. As this unstable equilibrium state is easily broken even by slight stimulation, water tends to move to a more stable state. That is, if a small piece of material is put into the supercooled liquid, or if the liquid is suddenly shaken, the liquid starts to be frozen at once such that its temperature reaches the freezing point, and maintains a stable equilibrium state at this temperature.
In general, an electrostatic atmosphere is made in a refrigerator and meat and fish are thawed in the refrigerator at a minus temperature. In addition to the meat and fish, fruit is kept fresh in the refrigerator.
This technology uses a supercooling phenomenon. The supercooling phenomenon indicates the phenomenon that a molten object or a solid is not changed although it is cooled to a temperature below the phase transition temperature in an equilibrium state. This technology includes Korean Patent Publication No. 2000-0011081 titled “Electrostatic field processing method, electrostatic field processing apparatus, and electrodes therefor”.
FIG. 1 is a view of an example of a conventional thawing and freshness-keeping apparatus. A keeping-cool room 1 is composed of a thermal insulator 2 and an outer wall 5. A mechanism (not shown) controlling a temperature inside the room 1 is installed therein. A metal shelf 7 installed in the room 1 has a two-layer structure. Target objects to be thawed or freshness-kept and ripened such as vegetables, meat and marine products are loaded on the respective layers. The metal shelf 7 is insulated from the bottom of the room 1 by an insulator 9. In addition, since a high voltage generator 3 can generate 0 to 5000 V of DC and AC voltages, an insulation plate 2a such as vinyl chloride, etc. is covered on the inside of the thermal insulator 2. A high-voltage cable 4 outputting the voltage of the high voltage generator 3 is connected to the metal shelf 7 after passing through the outer wall 5 and the thermal insulator 2.
When a user opens a door installed at the front of the keeping-cool room 1, a safety switch 13 (see FIG. 2) is turned off to intercept the output of the high voltage generator 3.
FIG. 2 is a circuit configuration view of the high voltage generator 3. 100 V of AC is supplied to a primary side of a voltage regulation transformer 15. Reference numeral 11 represents a power lamp and 19 a working state lamp. When the door 6 is closed and the safety switch 13 is on, a relay 14 is operated. This state is displayed by a relay operation lamp 12. Relay contact points 14a, 14b and 14c are closed by the operation of the relay 14, and 100 V of AC is applied to the primary side of the voltage regulation transformer 15.
The applied voltage is regulated by a regulation knob 15a on a secondary side of the voltage regulation transformer 15, and the regulated voltage value is displayed on a voltmeter. The regulation knob 15a is connected to a primary side of a boosting transformer 17 on the secondary side of the voltage regulation transformer 15. The boosting transformer 17 boosts the voltage at a ratio of 1:50. For example, when 60 V of voltage is applied, it is boosted to 3000 V.
One end O1 of the output of the secondary side of the boosting transformer 17 is connected to the metal shelf 7 insulated from the keeping-cool room 1 through the high-voltage cable 4, and the other end O2 of the output is grounded. Moreover, since the outer wall 5 is grounded, if the user touches the outer wall 5 of the keeping-cool room 1, he/she does not get an electric shock. Further, in FIG. 1, when the metal shelf 7 is exposed in the room 1, it should be maintained in an insulated state in the room 1. Thus, the metal shelf 7 needs to be separated from the wall of the room 1 (the air performs an insulation function). Furthermore, if a target object 8 is protruded from the metal shelf 7 and brought into contact with the wall of the room 1, the current flows to the ground through the wall of the room 1. Therefore, the insulation plate 2a is attached to the inner wall to prevent drop of the applied voltage. Still furthermore, when the metal shelf 7 is covered with vinyl chloride without being exposed in the room 1, an electric field atmosphere is produced in the entire room 1.
In the prior art, an electric field or a magnetic field is applied to the received object to be cooled, such that the received object enters a supercooled state. Accordingly, a complicated apparatus for producing the electric field or the magnetic field should be provided to keep the received object in the supercooled state, and the power consumption is increased during the production of the electric field or the magnetic field. Additionally, the apparatus for producing the electric field or the magnetic field should further include a safety device (e.g., an electric or magnetic field shielding structure, an interception device, etc.) for protecting the user from high power, when producing or intercepting the electric field or the magnetic field.
Japanese Patent Publication No. 2001-4260 discloses a supercooling control refrigerator which includes a temperature detection means and a control means controlling the temperature at a given set temperature in an openable/closable thermal insulation unit and which keeps the goods cold at a temperature below the freezing point during the supercooling operation. However, since the refrigerator controls the rotation number of a cool air circulation fan to adjust the temperature in the thermal insulation unit, if the temperature in the unit is reduced to a temperature below the set temperature, there is no means for raising the temperature to the set temperature within a short time. When the temperature in the unit is maintained at a temperature below the set temperature for a predetermined time, the goods intended to be stored in a supercooled state are frozen. In addition, the frozen goods cannot be thawed and stored again in the supercooled state. The refrigerator has low stability in maintaining a non-frozen state.
Korean Patent Registration No. 10-850062 describes a refrigerator having a space for receiving food and a storing room for cooling the space, the refrigerator including a cool air flowing space directly cooling the food receiving space and a thermal insulation layer insulating the cool air flowing space from the space, and storing the food in a supercooled state. However, if the temperature in the refrigerator is reduced to a temperature below a set temperature, there is no construction for raising the temperature. Therefore, the refrigerator also has low stability in maintaining a non-frozen state.
Japanese Patent Publication No. 2008-267646 discloses a refrigerator with a supercooling room which includes a freezing chamber with a temperature control means therein to continuously adjust the temperature between 0° C. and a temperature of a freezing temperature zone by stages, the supercooling room disposed in the freezing chamber and receiving the cool air from the freezing chamber, and a control apparatus controlling the freezing chamber so that the food stored in the supercooling room can be maintained in a supercooled state at a temperature below the freezing point without being frozen. The temperature of the freezing chamber or a switching chamber in which the supercooling room is installed is controlled to adjust the temperature of the supercooling room. The supercooling room is sealed with respect to the freezing chamber or the switching chamber such that a temperature change in the supercooling room is limited. However, when the food is stored in the supercooled state by slowing down the temperature change in the supercooling room by indirect cooling, it takes a long time for the food to reach the supercooled state. Moreover, if the temperature in the refrigerator is reduced to a temperature below a set temperature, there is no construction for raising the temperature. Accordingly, the refrigerator also has low stability in maintaining a non-frozen state.